In 1909, Aldo Leopold, a young officer with the United States Forest Service, had a transformative experience when he shot a mother wolf in Arizona's Apache National Forest. This moment left a lasting impression on him. In his essay "Thinking Like a Mountain," later included in his posthumous work Sand County Almanac, he reflected, “We never thought about not taking the opportunity to kill a wolf.” Watching the life fade from the wolf's eyes, he recognized a profound connection between her and the mountain that was foreign to him.

Leopold is now celebrated in ecological circles for his early insights into the relationship between wolves and deer populations, and how an overabundance of deer can adversely affect plant life. In "Thinking Like a Mountain," he articulated the concept of trophic cascades, which posits that apex predators are essential for maintaining ecosystem health. He argued that the removal of wolves and cougars in Arizona and other western regions would lead to unchecked deer populations that would devastate the forests. “Just as deer live in fear of wolves,” he noted, “mountains live in fear of deer.”

The Kaibab Plateau, near the Grand Canyon, was one of Leopold's study sites. By 1924, the deer population there had skyrocketed to 100,000, but soon after, a severe crash occurred, with 60% of the deer dying from starvation between 1924 and 1926. Leopold foresaw that similar patterns would emerge across the United States wherever predators had been eradicated. By 1920, wolves and cougars had largely vanished from east of the Mississippi River due to hunting and habitat fragmentation, and were on the brink of extinction in many western areas. Conservationists responded by regulating deer hunting and advocating for their reintroduction in the eastern U.S., while also establishing protected areas in state and federal forests.

However, contrary to Leopold's expectations, deer populations did not collapse under overconsumption. Instead, they adapted by modifying their habitats. Recent studies have shown that certain grasses preferred by deer can rebound swiftly after being over-browsed, much like suburban lawns after mowing. Consequently, white-tailed deer populations in the U.S. have soared to approximately 32 million, the highest levels recorded.

One biologist remarked that deer, similar to humans, can drastically reduce biodiversity without immediately suffering the consequences. In 1968, Stanford professor Paul Ehrlich predicted that overpopulation would lead to the extinction of another species, yet he was mistaken. Like deer, humans have adapted their environments, particularly arable land, to enhance their survival. As humanity flourishes across the globe, it may be worth reflecting on the rampant deer populations.

Conservation biologists have expressed concerns regarding the unchecked growth of deer populations. Many advocate for culling measures to mitigate the ecological damage caused by overpopulation. For instance, in 2011, the National Park Service and the U.S. Department of Agriculture collaborated with hunters to "harvest" 3 million antlerless deer. Thomas Rooney, a leading researcher on deer populations, described the situation as an "apocalypse."

During a visit to Rooney's lab in Ohio, I observed a well-used copy of Ehrlich's The Population Bomb. When I inquired about parallels between human and deer overpopulation, he responded, “Deer, like humans, can come in and eliminate biodiversity, though not to their immediate detriment.”

We ventured into a 200-acre biological preserve on campus, where the sun filtered through the trees, creating a picturesque scene. However, Rooney warned that the forest was in peril. Many native tree saplings—such as shingle oaks, walnuts, and ashes—were struggling or dying due to excessive deer browsing. He emphasized that without population control, the forest would fail to regenerate.

Kneeling, Rooney pointed out a white ash sapling, reduced to a few stubs. “Deer ruins,” he stated. Nearby, a struggling elm sapling stood barely a foot tall, with the potential to reach only three feet. We also found wild ginger, a plant that deer had just begun to graze upon, indicating their relentless march toward consuming every available resource.

Rooney shared findings from a recent Conservation Biology study that lamented the “pandemic deer overabundance,” suggesting that deer populations act as a blight on ecosystems. Damage is evident at about 15 deer per square mile, worsening at higher densities. Some northeastern regions report deer populations reaching 100 per square mile, while the Wright State preserve has about 40 per square mile. He referred to a 2013 article co-authored by Nature Conservancy scientists, asserting that no current threat to forest habitats exceeds the danger posed by deer.

Research from Cornell University indicated that one-third of New York's forests have suffered so extensively from over-browsing that new saplings are not taking root, leaving only older trees behind. A 2005 study from British Columbia revealed that predator-free islands, where deer had thrived for over half a century, had 85% fewer shrubs and herbaceous plants compared to islands without deer. In Quebec's Anticosti Island, the introduction of deer led to the extinction of native bears, reliant on the fruits the deer consumed. This phenomenon, where a prey species eradicates its predator through resource depletion, is striking. Moreover, studies in Pennsylvania have shown that songbird populations plummeted due to deer destruction of their nesting understory.

Curious about deer’s ability to flourish while simultaneously simplifying ecosystems, I asked Rooney about his research in Wisconsin, where he discovered that just a few grass species could survive under high deer densities, leading to the loss of about 80% of native plant species. “This represents the loss of 300 million years of evolutionary history,” he lamented.

As we left the forest, the sound of a turkey vulture echoed overhead, while a military plane approached a nearby air force base. Upon exiting the woods, we returned to the university's urban landscape, where human activity dominated. The manicured lawns and paved surfaces offered little in the way of biodiversity, save for a few squirrels. This environment starkly contrasted the vibrant ecosystems of the past.

A few days later, I spoke with Leon Kolankiewicz, a former U.S. Fish and Wildlife biologist, about the impact of 7 billion humans on wildlife. He noted that habitats are being destroyed and fragmented, while ecosystems face pollution and invasive species. Biologist E.O. Wilson has pointed out that Earth is currently experiencing its sixth mass extinction, with extinction rates far outpacing the emergence of new species.

Kolankiewicz suggested a paradoxical hope for humanity amidst this ecological crisis. He argued that biodiversity loss might not endanger our survival. “Hypothetically, we could persist for a while on a simplified planet,” he posited, drawing a parallel to deer. Throughout history, humans have invaded and altered ecosystems, leading to biodiversity loss while continuing to thrive.

Reflecting on human history, one could consider the arrival of humans in North America around 13,000 years ago, where they quickly decimated megafauna, thus securing their dominance over a now predator-less landscape. The rise of agricultural civilizations, from the Mayans to ancient Egypt, showcases how high-density populations exploited resources, causing significant biodiversity loss, only to eventually face collapse due to overexploitation.

The optimist might argue that humanity has transcended the limits of carrying capacity. If the human population doubled from 1804 to 1927, then again by 1974, reaching nearly 7 billion today with projections of over 10 billion by 2100, technological advancements like genetically modified crops and renewable energy sources could stave off collapse. Geoffrey West, a physicist, noted that innovation often resets the clock, but the question remains: how long can this clock keep ticking? History has repeatedly shown Malthusian pessimists to be mistaken. Like deer without predators, humanity has reshaped its environment to its advantage.

Weeks later, I visited my family’s vacation cabin in the Catskill Mountains. This rustic dwelling, lacking modern conveniences, offered a glimpse into a simpler time. I used to escape the city to immerse myself in nature, believing I was witnessing a thriving ecosystem.

When my mother purchased the property in 1971, it was rich with diverse tree species. Today, however, many of these trees are dying, and the once-dense forest is now sparse and easy to navigate. The vibrant ecosystem I once cherished has been drastically altered, echoing Rooney’s sentiment that “home for the deer is now a lot less interesting.” This reality resonates with us as well.

Christopher Ketcham is a contributing editor at Harper’s Magazine.

Originally published at Nautilus on December 11, 2014.